Nonbursting electrical capacitor



United States Patent Otice A 3,304,473 NONBURSTING ELECTRICAL CAPACITORPaul H. Netherwood,.Livingston L. Rice, and David A.

Hynes, Williamstown, Mass., assignors to Sprague Electric Company, NorthAdams, Mass., a corporation of Massachusetts Filed Nov. 12, 1963, Ser.No. 322,761 1 Claim. (Cl. 317-256) The present invent-ion is concernedwith electrical capacitors and more particularly with lhermeticallysealed capacitors which tend to generate suiicient interna-l gaspressure to burst the units.

The dangers of Va bursting capacitor are well-known to those skilled inthe art and need not be elaborated upon.

It is an object of this invention to eliminate the dangers attendant thebursting of a liquid dielectric-impregnated rolled capacitor.

This and other objects and advantages of the present invention will bemade obvious to those skilled in the art by the following descriptionwhen considered in relation to the accompanying drawing, of which:

FIGURE l is a side-view, partly 4in section of a conventionalhermetically sealed, liquid dielectric-impregnated, rolled capacitor;

FIGURE 2 is a side-view, partly in section, of one modification of thepresent invention;

FIGURE 3 is a perspective view, partly broken away, of the capacitorsection of FIGURE 2;

FIGURE 4 is a side-view, partly in section, of another modification ofthe present invention; and

FIGURE 5 is `a side-view, partly in section, of the capacitor of FIGURE4 after being subjected to internal pressure.

FIGURE 1 illustrates a conventional hermetically sealed capacitorcomprising a can containing a rolled section 11; conductive tabs orleads 12; insulating spacer 13; can cover 14 and terminals 15. Theconductive tabs 12 are soldered, riveted or welded to the bottom ofterminals 15 as shown at 16. The capacitor is filled with a liquiddielectric. 4

It will be noted that :tabs 12 in FIGURE l have considerable slackbetween the top of section 11 and the connections 16 at the bottom ofthe terminals. The reason that this slack is present in the capacitor isthat lthe welding, soldering or riveting of the tabs to the terminals isaccomplished by reaching under the can cover with the soldering, weldingor riveting device and making the connection. Because of the physicalsize of the standard soldering, welding and riveting devices and thedesire to actually see the connection made, it is, therefore, neces*sary to use a longer tab than is actually required for electricalcommunication.

An internal short in a capacitor .or this type is one condition whichresults in the rapid generation of gas within the unit. When this occursthe can bulges in all directions with a considerable doming of the cancover. When .the capacitor bursts, the point `of opening is cornmonly atthe seam joining the can cover and the can.

The objects of the present invention Iare attained by ernploying thebulging or doming o-f the can cover as the means for disconnecting atleast one tab from its terminal connection and thereby stopping yfurtherrapid generation of pressure. This is accomplished lby eliminating, fromat least one tab, the tab slack between the top of the capacitor sectionand the tab connection at the bottom of the terminals and providingmeans which will restrain movement of the capacitor section during apressure build-up in the can.

The tab slack is eliminated by employing special small PatentedFeb. 14,1967 size welding, soldering or riveting heads. These heads form no partof the present invention. As a result of the use of such a head there isno appreciable slack or excess tab between the section and theterminals. In combination with this substantially slack-free tab,- asection-restraining me-ans cooperates to protect the unit from bursting.rllhe sectionrestraining means can be any element or arrangement whichprevents the section from being pulled in the direction of the can coverduring a build-up of pressure within the can. Examples of such meansare, an internal beading in the can wall at a point adjacent the end ofthe section nearest the can cover; one or more strips of relativelyinflexible insulating ma- -teri-al extending from within the capacitorsection near the periphery thereof to the underside of the can cover; asplit ring, a collar or pair of semicircles of comparatively inexibleinsulating material positioned between the section and the can cover,etc.

The invention is best illustrated by reference to FIG- URES 2, 3, 4 and5. FIGURE 2 has all of the essential capacitor elements of FIGURE l andin addition it shows that tabs 12 have no appreciable slack between thebottom of terminals 15 and the top of rolled section 11. The other endof tab 112, i.e. the end making contact with the section, although notshown, is 4firmly aixed as by a weld or stitches to the section. Thisprevents the tabs from moving freely within the section and insurestension on the terminal connections when internal pressure develops.

Extension members 17 protrude from capacitor section 11 and extend tothe bottom of can cover 14. FIGURE 3 Iis a perspective view of section11 of FIGURE 2, which more clearly shows the relative lpositions ofextension members 17. When internal pressure begins to build-'up withinthe unit of FIGURE 2, as it will when an electrical short circuitdevelops, can cover 14 will begin to bulge or dome. As the domingincreases the entire section 11 will tend to be pulled in the directionof the can cover. This movement will be restrained by extension members17 and =as the doming continues, at least one tab 12 will be torn freeof the bottom of terminals 15. This will interrupt the electric circuit,preventing .any further rapid build-up of gas pressure.

In the modification of FIGURE 4 instead of having extension member 17 asthe section restraining means, an internal beading 18 in the wall of can10 at a point adjacent the end of the section is employed to preventmovement of the section. FIGURE 5 shows this unit after :internalpressure has bu-lged the can cover causing tabs 12 to be disconnectedfrom terminals 15 at point 19.

Another section restraining means which may be ernployed is a collar ora ring which is split. This means may be positioned between the cancover and the end of the section after the tab connections yare made. Asinternal pressure builds up it will restrain section 11 in the samemanner as extension 17 of FIGURE 2.

The restraining collar and the extension members 17 can be made of anymaterial which will not yield under the force exerted by the doming ofthe can cover. Any one of a number of commercial plastics or lresins maybe employed. Wood, stiff paper or cardboard may also be used. In anycase the material selected must be compatible with the other elements ofthe capacitor.

As an aid in insuring that the tabs or leads will be disconnected in theevent of .a rapid build-up of pressure the tabs or leads may be nickedor notched. This then will be the weakest point in the leads or tabs andthe point of severance.

As yis evident from the foregoing the invention is not to be limited tothe rather specic illustrative device. Modications and variations, aswell as the substitution of equivalents may be made Without departingfrom the spirit of the invention as defined in the appended claims.

What is claimed is:

An electrical capacitor consisting essentially of a capacitor section inya hevmetically sealed can, said can having a cover domable in responseto linc-reasing internal pressure; conductive tabs irmly affixed to saidseccion; said section being connected to terminals in said cover viasaid conductive tabs, lat least one of said tabs being substantiallyslack-free; at least one strip of relatively ineXible insulatingmaterial extending from Within said capacitor section at a point nearthe periphery thereof iand extending to the underside of said cover,said strip functioning as a section restraining means xedly locatingsaid section in said can; whereby progressive doming of the cover, inresponse to increasing internal pressure, disrupts electricalcommunication between said slack-free tab and its terminal therebystopping further rapid generation of pressure 4before bursting of thecapacitor.

References Cited by the Examiner UNITED STATES PATENTS 2,029,430 2/1936Kopinski 317-260 2,896,049 7/1959 Maier l3-256 X 3,182,238 5/1965 Toder317-260 3,221,225 1l/l965 Sternbeck 317-256 FOREIGN PATENTS 1,298,073 5/1962 France.

LEWIS H. MYERS, Primary Examiner.

15 JOHN F. BURNS, ROBERT K. SCHAEFER,

LARAMIE E. ASKIN, Examiners.

E. GOLDBERG, Assistant Examiner.

